IOWA ACADEMY OP SCIENCES. 
109 
vicinity of streams is consistent with the theory of loess-forma- 
tion presented by the writer before this academy.* 
Plants, and especially forests, develop chiefly and prima- 
rily along streams. This creates conditions favorable to land- 
molluscs, and at the same time forms a trap for the dust, 
carried from adjacent, more barren regions. The occurrence of 
loess in the eastern part of Iowa, chiefly along the border of the 
Iowan drift sheet, may also be explained on the same ground. 
After the melting of the ice, the terminal moraines offered the 
first lodging-place for plants. Here, forests early developed, 
and the conditions for entrapping the dust from adjacent, less- 
favored territory, which was probably dry during a part of the 
year, were here first developed We are in the habit of describ- 
ing the lobed ridges of loess-regions as characteristic of loess 
topography, yet they are quite as much characteristic of some 
drift-areas; for example, along the Big Sioux river in Iowa and 
South Dakota. In eastern Iowa the surface of the loess is 
largely shaped by the underlying moraines, which first 
presented conditions suitable to the deposition of the loess, 
and where, consequently, the deposit is best developed. The 
loess at Natchez does not show this “ loess-topography in the 
same degree ” 
Third. — The depauperation of some forms of shells, and the 
presence of others which are normally inhabitants of dry 
regions, suggest a climate sufficiently dry that, during a part 
of the year, at least, clouds of dust could be taken up by 
the winds. 
Fourth. — The overwhelming preponderance of land- snails 
in the loess must always be borne in mind. This, however, 
does not prove that the loess regions were entirely devoid 
of lakes and streams, but rather that the loess proper was 
deposited chiefly upon higher grounds, for, if by any agency 
fine material was uniformly deposited over all of Iowa to-day, 
covering the successive generations of our present molluscan 
fauna, there would be a much greater proportion of aquatic 
and moisture-loving species than we find any where in the loess. 
Fifth . — The amount of material carried by the winds need not 
have been so great as is sometimes assumed. The estimate 
made by the writerf for the rate of deposition for east- 
ern loess (1 mm. per year), and that made by Keyes| 
*Proc. Iowa Acad. Sci., Vol. Ill, p. 82 et seg. 
tProc. Iowa Acad. Sci., Vol. Ill, p. 88. 
•4: Am. Jour, of Sci., (4), Vol. VI, pp. 301-302. 
